Use Clean
Create 5 courses:
no enrolled users
1 enrolled user, 0 activities
1 enrolled user, 10+ gradeable activities
100 enrolled users, 0 activities
100 enrolled users, 10+ gradeable activities
Go to the grade report, and make sure the floading headers work as you'd expect
Note: There are no floating headers when there are no enrolled users, just check nothing breaks when this is the case
Repeat on theme_base
Repeat on all supported browsers